What is the third person reflective pronoun? I have found both "si" and "mem" to mean "himself/herself/itself." Is there a difference between these two words?

The only thing I notice is that "mem" does not include "themselves." Is that the only difference? Am I missing something? Is there perhaps a better time to use either word?

Dankon.

The reflexive pronoun for the third person of both singular and plural is "si/sin".

For the use of "mem", please see my answer to your other post.

"Mem" works with all pronouns ("themselves".)

They built the house themselves.
Ili mem konstruis la domon.

"Mem" also works with nouns.
La urbestro mem konstruis la magistratan domon.